The NIH Animal Genetic Resource (NIHAGR) maintains several animal species, and many invaluable genotypes within a species, which are used as models in biomedical research. To protect these animal models, an embryo bank was established as an economical means to indefinitely preserve these stocks and strains of rodents and rabbits. Cryopreserving embryos ensures the availability of a genotype by safeguarding it against loss due to disease, reproductive failure, natural catastrophe or genetic contamination. The embryo cryobank minimizes changes due to genetic drift by providing a resource from which to periodically rederive defined genotypes for restandardization or testing of foundation breeding colonies. During the past year, for example, embryos banked in 1984 (B6e(84)/N and C3e(84)/N) were used to produce live animals for use in routine genetic monitoring by skin grafting. These animals, from a much earlier breeding generation, were used as controls for testing genetic drift in current NIHAGR C57BL/6N and C3H/HeN foundation colonies. As a recognized repository for laboratory animal species, the NIHAGR routinely provides breeding pairs upon request throughout the biomedical research community. Rather than shipping live animals, cryopreserved embryos represent a simpler, more humane and easier method to transport requested models worldwide. Another important benefit of embryo handling, cryopreservation, and transfer procedures is to eliminate the vertical transmission of some murine diseases. Genotype and species differences continue to impact on the efficiency of this effort.